External Charger for an Implantable Medical Device Having at Least One Sense Coil Concentric with a Charging Coil For Determining Position

ABSTRACT

A charging system for an Implantable Medical Device (IMD) is disclosed having a charging coil and one or more sense coils. The charging coil and one or more sense coils are preferably housed in a charging coil assembly coupled to an electronics module by a cable. The charging coil is preferably a wire winding, while the one or more sense coils are concentric with the charging coil and preferably formed in one or more traces of a circuit board. The magnitude of one or more voltages induced on the one or more sense coils can be measured to determine the position of the charging coil relative to the IMD, and in particular whether the charging coil is (i) centered, (ii) not centered but not misaligned, or (iii) misaligned, with respect to the IMD being charged, which three conditions sequentially comprise lower coupling between the charging coil and the IMD.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/350,451, filed Jun. 15, 2016, which is incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to wireless external chargers for use inimplantable medical device systems.

BACKGROUND

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder subluxation, etc.The description that follows will generally focus on the use of theinvention within a Spinal Cord Stimulation (SCS) system, such as thatdisclosed in U.S. Pat. No. 6,516,227. However, the present invention mayfind applicability in any implantable medical device system, including aDeep Brain Stimulation (DBS) system.

As shown in FIGS. 1A-1C, a SCS system typically includes an ImplantablePulse Generator (IPG) 10 (Implantable Medical Device (IMD) 10 moregenerally), which includes a biocompatible device case 12 formed of aconductive material such as titanium for example. The case 12 typicallyholds the circuitry and battery 14 (FIG. 1C) necessary for the IMD 10 tofunction, although IMDs can also be powered via external RF energy andwithout a battery. The IMD 10 is coupled to electrodes 16 via one ormore electrode leads 18, such that the electrodes 16 form an electrodearray 20. The electrodes 16 are carried on a flexible body 22, whichalso houses the individual signal wires 24 coupled to each electrode. Inthe illustrated embodiment, there are eight electrodes (Ex) on each lead18, although the number of leads and electrodes is application specificand therefore can vary. The leads 18 couple to the IMD 10 using leadconnectors 26, which are fixed in a non-conductive header material 28,which can comprise an epoxy for example.

As shown in the cross-section of FIG. 1C, the IMD 10 typically includesa printed circuit board (PCB) 30, along with various electroniccomponents 32 mounted to the PCB 30, some of which are discussedsubsequently. Two coils (more generally, antennas) are show in the IMD10: a telemetry coil 34 used to transmit/receive data to/from anexternal controller (not shown); and a charging coil 36 for charging orrecharging the IMD's battery 14 using an external charger, which isdiscussed in detail later.

FIG. 2 shows the IMD 10 in communication with an external charger 50used to wirelessly convey power to the IMD 10, which power can be usedto recharge the IMD's battery 14. The transfer of power from theexternal charger 50 is enabled by a primary charging coil 52. Theexternal charger 50, like the IMD 10, also contains a PCB 54 on whichelectronic components 56 are placed. Again, some of these electroniccomponents 56 are discussed subsequently. A user interface 58, includingtouchable buttons and perhaps a display and a speaker, allows a patientor clinician to operate the external charger 50. A battery 60 providespower for the external charger 50, which battery 60 may itself berechargeable. The external charger 50 can also receive AC power from awall plug. A hand-holdable housing 62 sized to fit a user's handcontains all of the components.

Power transmission from the external charger 50 to the IMD 10 occurswirelessly and transcutaneously through a patient's tissue 25, viainductive coupling. FIG. 3 shows details of the circuitry used toimplement such functionality. Primary charging coil 52 in the externalcharger 50 is energized via charging circuit 64 with an AC current,Icharge, to create an AC magnetic charging field 66. This magnetic field66 induces a current in the secondary charging coil 36 within the IMD10, providing a voltage across coil 36 that is rectified (38) to DClevels and used to recharge the battery 14, perhaps via a batterycharging and protection circuitry 40 as shown. The frequency of themagnetic field 66 can be perhaps 80 kHz or so. When charging the battery14 in this manner, is it typical that the housing 62 of the externalcharger 50 touches the patient's tissue 25, perhaps with a chargerholding device or the patient's clothing intervening, although this isnot strictly necessary.

The IMD 10 can also communicate data back to the external charger 50during charging using reflected impedance modulation, which is sometimesknown in the art as Load Shift Keying (LSK). This involves modulatingthe impedance of the charging coil 36 with data bits (“LSK data”)provided by the IMD 10's control circuitry 42 to be serially transmittedfrom the IMD 10 to the external charger 50. For example, and dependingon the logic state of a bit to be transmitted, the ends of the coil 36can be selectively shorted to ground via transistors 44, or a transistor46 in series with the coil 36 can be selectively open circuited, tomodulate the coil 36's impedance. At the external charger 50, an LSKdemodulator 68 determines whether a logic ‘0’ or ‘1’ has beentransmitted by assessing the magnitude of AC voltage Vcoil that developsacross the external charger's coil 52 in response to the chargingcurrent Icharge and the transmitted data, which data is then reported tothe external charger's control circuitry 72 for analysis. Such backtelemetry from the IMD 10 can provide useful data concerning charging tothe external charger 50, such as the capacity of the IMD's battery 14,or whether charging of the battery 14 is complete and operation of theexternal charger 50 and the production of magnetic field 66 can cease.LSK communications are described further for example in U.S. PatentApplication Publication 2013/0096652.

External charger 50 can also include one or more thermistors 71, whichcan be used to report the temperature (expressed as voltage Vtherm) ofexternal charger 50 to its control circuitry 72, which can in turncontrol production of the magnetic field 66 such that the temperatureremains within safe limits. See, e.g., U.S. Pat. No. 8,321,029,describing temperature control in an external charging device.

Vcoil across the external charger's charging coil 52 can also beassessed by alignment circuitry 70 to determine how well the externalcharger 50 is aligned relative to the IMD 10. This is important, becauseif the external charger 50 is not well aligned to the IMD 10, themagnetic field 66 produced by the charging coil 52 will not efficientlybe received by the charging coil 36 in the IMD 10. Efficiency in powertransmission can be quantified as the “coupling” between thetransmitting coil 52 and the receiving coil 36 (k, which ranges between0 and 1), which generally speaking comprises the extent to which powerexpended at the transmitting coil 52 in the external charger 50 isreceived at the receiving coil 36 in the IMD 10. It is generally desiredthat the coupling between coils 52 and 36 be as high as possible: highercoupling results in faster charging of the IMD battery 14 with the leastexpenditure of power in the external charger 50. Poor coupling isdisfavored, as this will require high power drain (e.g., a high Icharge)in the external charger 50 to adequately charge the IMD battery 14. Theuse of high power depletes the battery 60 in the external charger 50,and more importantly can cause the external charger 50 to heat up, andpossibly burn or injure the patient.

Generally speaking, if the external charger 50 is well aligned with theIMD 10, then Vcoil will drop as the charging circuitry 64 provides thecharging current Icharge to the charging coil 52. Accordingly, alignmentcircuitry 70 can compare Vcoil, preferably after it is rectified 76 to aDC voltage, to an alignment threshold, Vt. If Vcoil<Vt, then externalcharger 50 considers itself to be in good alignment with the underlyingIMD 10. If Vcoil>Vt, then the external charger 50 will consider itselfto be out of alignment, and can indicate that fact to the patient sothat the patient can attempt to move the charger 50 into betteralignment. For example, the user interface 58 of the charger 50 caninclude an alignment indicator 74. The alignment indicator 74 maycomprise a speaker (not shown), which can “beep” at the patient whenmisalignment is detected. Alignment indicator 74 can also oralternatively include one or more Light Emitting Diodes (LED(s); notshown), which may similarly indicate misalignment.

Charger-to-IMD coupling depends on many variables, such as thepermeability of the materials used in the external charger 50 and theIMD 10, as well materials inherent in the environment. Coupling is alsoaffected by the relative positions of the external charger 50 and IMD10, as shown in FIGS. 4A-4C. For best coupling (higher values of k), itis preferred that axes around which coils 52 and 36 are wound (52′ and36′) are parallel and collinear, with the coils 52 and 36 as close aspossible (d1) to each other, as shown in FIG. 4A. Distance d1 indicatesthe depth between the external charger 50 and the IMD 10, and isgenerally constant given that the external charger is generally placedon the patient's tissue 25, and that the IMD 10 has been implanted at aparticular depth. Deviations from these ideal conditions will generallyreduce coupling, as shown in FIGS. 4B-4C. In FIG. 4B for instance, thecoil axes 52′ and 36′ are not collinear, but instead are laterallyoffset (x). In FIG. 4C, the coil axes 52′ and 36′ are parallel andcollinear, but the IMD 10 is relatively deep (d2). In any of thesenon-ideal cases, coupling will be reduced, meaning that the IMD'sbattery 14 will not charge as quickly, or that the external charger 50must output more power (e.g., Icharge must be higher) to affect the samecharging rate of the IPG's battery 14.

It should be noted with reference to FIG. 4C that the depth d2 of theIMD 10 cannot generally be changed, as this parameter results from howthe IMD 10 was implanted in the patient. As a result, the externalcharger 50 may be in alignment with the IMD 10, even if the couplingbetween the external charger 50 and the IMD 10 is relatively poor (andthus Vcoil is relatively high). It can be useful to adjust the alignmentthreshold Vt (i.e., upwards) used by the alignment circuitry 70 in suchcases so that the external charger 50 will not unreasonably indicatemisalignment to the patient when there is nothing the patient can do toimprove alignment. U.S. Pat. No. 9,227,075 describes one technique foradjusting Vt to address alignment as a function of implant depth,although this technique is not described here.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C show different views of an implantable pulse generator, atype of implantable medical device (IMD), in accordance with the priorart.

FIG. 2 shows an external charger being used to charge a battery in anIMD, while

FIG. 3 shows circuitry in both, in accordance with the prior art.

FIGS. 4A-4C show various position between an external charger and an IMDthat can affect their coupling, in accordance with the prior art.

FIGS. 5A-5E show an improved charging system having a charging coilassembly and an electronics module, with FIGS. 5B-5E showing use of oneor more sense coils with a charging coil in the charging coil assembly,in accordance with examples of the invention.

FIGS. 6A-6C show use of a first alignment sense coil of a constantradius in the charging coil assembly, as well as circuitry for detectingand indicating misalignment between the charging coil and the IMD, inaccordance with an example of the invention.

FIGS. 7A-7C show use of a second alignment sense coil having edgedetection capability in the charging coil assembly, as well as circuitryfor detecting and indicating misalignment between the charging coil andthe IMD, in accordance with an example of the invention.

FIGS. 8A and 8B show use of a third alignment sense coil arrangementcomprising two or more separate alignment sense coils in the chargingcoil assembly, as well as circuitry for detecting and indicatingmisalignment between the charging coil and the IMD, in accordance withan example of the invention.

FIGS. 9A-9C show use of a centering sense coil in the charging coilassembly, as well as circuitry for detecting and indicating anon-centered condition between the charging coil and the IMD, inaccordance with an example of the invention.

FIGS. 10A and 10B show use of both an alignment sense coil and acentering sense coil in the charging coil assembly, as well as circuitryfor detecting and indicating misalignment and/or non-centered conditionsbetween the charging coil and the IMD, in accordance with an example ofthe invention.

FIGS. 11A and 11B show use of a single alignment/centering sense coil inthe charging coil assembly, as well as circuitry for detecting andindicating misalignment and/or non-centered conditions between thecharging coil and the IMD, in accordance with an example of theinvention.

FIG. 12A shows an algorithm operable in the charging system fordetermining alignment and centering, and for controlling the magneticfield produced by the charging coil, while FIG. 12B shows the magneticfield produced via the algorithm, in accordance with an example of theinvention.

FIG. 13A shows modified position circuitry for the charger system ableto determine IMD-to-charger positioning using one of more of the sensecoil parameters of magnitude, phase angle, and resonant frequency, whileFIGS. 13B-13D show circuitry and manners in which these parameters canbe determined or measured, in accordance with an example of theinvention.

FIGS. 14A and 14B show contours of experimentally-determined datarelating the sense coil parameters of magnitude, phase angle, andresonant frequency to charger-to-IMD radius and depth, as useful in themodified position circuitry of FIG. 13A, in accordance with an exampleof the invention.

FIG. 15A shows power circuitry for the charger system able to adjustmagnetic field power using one of more of the sense coil parameters ofmagnitude, phase angle, and resonant frequency, while FIG. 15B showsexperimentally how charger-to-IMD position affects power received at theIMD, in accordance with an example of the invention.

FIG. 16 shows an integrated external charger in which the electronics,charging coil, and sense coil(s) are housed in a single housing, inaccordance with an example of the invention.

FIGS. 17A-17E show the use of additional sense coils arranged in mannersto provide information regarding one or more direction by which thecharging coil 126 is misaligned or non-centered with respect to an IMD,in accordance with examples of the invention.

FIG. 18 shows how one or more sense coils in the charging system can beactively driven to determine charger-to-IMD positioning and/or howmagnetic field power can be adjusted, in accordance with an example ofthe invention.

FIG. 19 shows how charger-to-IMD positioning and/or magnetic field poweradjustment can be assisted by the provision of hardware enabling the IMDto telemeter a coupling parameter to the charging system, in accordancewith an example of the invention.

DETAILED DESCRIPTION

An improved charging system 100 for an IMD 10 is shown in FIG. 5A.Charging system 100 includes two main parts: an electronics module 104and a charging coil assembly 102 which includes a charging coil 126. Theelectronics module 104 and the charging coil assembly 102 are connectedby a cable 106. The cable 106 may be separable from both the electronicsmodule 104 and the charging coil assembly 102 via a port/connectorarrangement, but as illustrated cable 106 is permanently affixed to thecharging coil assembly 102. The other end of the cable 106 includes aconnector 108 that can attach to and detach from a port 122 of theelectronics module 104.

Electronics module 104 preferably includes within its housing 105 abattery 110 and active circuitry 112 needed for charging systemoperation, some of which are described subsequently. Electronics module104 may further include a port 114 (e.g., a USB port) to allow itsbattery 110 to be recharged in conventional fashion, and/or to allowdata to be read from or programmed into the electronics module, such asnew operating software. Housing 105 may also carry a user interface,which as shown in the side view of FIG. 5B can include an on/off switchto begin/terminate generation of the magnetic field 66, and one or moreLEDs 118 a and 118 b. In one example, LED 118 a is used to indicate thepower status of the electronics module 104. For example, LED 118 a maybe lit when its battery 110 is charged, and may blink to indicate thatthe battery 110 needs charging. LED 118 b may operate as explainedfurther below. More complicated user interfaces, such as thoseincorporating a speaker and a display, could also be used. Userinterface elements can be included on other faces of the electronicmodule's housing 105, and may be placed such that they are easily viewedfor the therapeutic application at hand (e.g., SCS, DBS). Electronicsare integrated within the housing 105 of the electronics module 104 by acircuit board 120.

Charging coil assembly 102 preferably contains only passive electroniccomponents that are stimulated or read by active circuitry 112 withinthe electronics module 104. Such components include the primary chargingcoil 126 already mentioned, which as illustrated comprises a winding ofcopper wire and is energized by charging circuitry 64 (FIG. 6A) in theelectronics module 104 to create the magnetic charging field 66 thatprovides power to the IMD 10, such as may be used to recharge the IMD10's battery 14. Further included within the charging coil assembly 102are one or more sense coils. As explained in detail later, the one ormore sense coils are measured in various ways to perform differentfunctions in the charging system 100. For example, sense coilmeasurements can be used to determine the position of the charging coil126 (charging coil assembly 102) with respect to the IMD 10 beingcharged, and more specifically whether the charging coil 126 is alignedand/or centered with respect to an IMD 10 being charged. Sense coilmeasurements can also be used to adjust the power of the magnetic field66 provided by the charging coil 126.

As shown in the cross section of FIG. 5B, the one or more sense coilsare preferably formed using one or more traces in a circuit board 124,which circuit board 124 is also used to integrate the electroniccomponents within the charging coil assembly 102. Circuit board 124 isshown in isolation in FIG. 5C. While it is preferred that charging coil126 comprise a wire winding, and that the one or more sense coilscomprise traces within the circuit board 124, this is not strictlynecessary: the charging coil 126 can also be formed from traces incircuit board 124, and the one or more sense coils can comprise wirewindings. Note that the charging coil 126 and the one or more sensecoils, as well as being concentric, are also formed in planes that areparallel, and can also be formed in the same plane as discussed furtherbelow with respect to FIG. 5D.

Further passive components preferably included within the charging coilassembly 102 include one or more tuning capacitors 131. As shown inlater circuit diagrams (e.g., FIG. 6A), a capacitor 131 is coupled tothe charging coil 126 to tune the resonant frequency of this L-C circuit(e.g., to 80 kHz). One skilled in the art will understand that the valueof the capacitor 131 (C) connected to the charging coil 126 will bechosen depending on the inductance (L) of that coil, in accordance withthe equation f(res)=1/sqrt(2πLC). Each of the one or more sense coilsmay also be coupled to a tuning capacitor 131, although this is notnecessary and is not shown in further circuit diagrams. A tuningcapacitor 131 can be placed in series or in parallel with its associatedcoil, although a series configuration is shown in subsequent figures.

The charging coil assembly 102 can further include one or moretemperature sensors, such as thermistors 136, which can be used toreport the temperature of the charging coil assembly 102 to theelectronics module 104 (FIG. 6A, Vtherm). Such temperature data can inturn control production of the magnetic field 66 such that thetemperature remains within safe limits. See, e.g., U.S. Pat. No.8,321,029, describing temperature control in an external chargingdevice.

Electronic components within the charging coil assembly 102 can beintegrated differently. In FIGS. 5B and 5C, a single circuit board 124is used, with the charging coil 126 mounted to the patient-facing sideof the circuit board 124, and with wires 134 in the cable 106 preferablycoupled to the circuit board 124. In FIG. 5D however, two circuit boards124 a and 124 b are used. Circuit board 124 b is outside of the area ofthe charging coil 126, and includes capacitors 131. Circuit board 124 ais within the area of the charging coil 126, and includes the one ormore sense coils and the thermistors 136. In the two-circuit-board 124 aand 124 b arrangement of FIG. 5D, notice in the cross section that thecharging coil 126 and circuit boards 124 a and 124 b can be generallylocated in the same plane, which allows for a thinner construction ofthe charging coil assembly 102. In FIG. 5D, the wires 134 within thecable 106 can connect to both circuit boards 124 a and 124 b to allowcommunication between the components and the electronics module 104. Thetwo circuit boards 124 a and 124 b can also have connections betweenthem (not shown).

Components in the charging coil assembly 102 are integrated within ahousing 125, which may be formed in different ways. In one example, thehousing 125 may include top and bottom portions formed of hard plasticthat can be screwed, snap fit, ultrasonic welded, or solvent bondedtogether. Alternatively, housing 125 may include one or more plasticmaterials that are molded over the electronics components. One side ofthe housing 125 may include an indentation 132 to accommodate thethickness of a material (not shown) that can be useful to affixing thecharging coil assembly 102 to the patient, to the patient's clothes, orwithin a holding device such as a charging belt or harness. See, e.g.,U.S. Patent Application Publication 2016/0301239, disclosing a belt forholding a charging coil assembly and control module that can be usedwith charging system 100. Such material may include Velcro ordouble-sided tape for example.

FIG. 5E shows another example of charging coil assembly 102. Thisexample shows the one or more thermistors 136 on the top of the PCB 124along with the charging coil 126 and other components, such as the oneor more tuning capacitors 131. To assist with temperature detection, athermal diffuser 123 is included, as disclosed in U.S. ProvisionalPatent Application Ser. No. 62/514,304, filed Jun. 2, 2017. The thermaldiffuser 123 is shown within and in contact with the charging coil 126,but could cover the charging coil 126 or appear outside of the chargingcoil 126 as well. As its name implies, thermal diffuser 123 helps toconduct heat generated by excitation of the charging coil 126 andgeneration of the magnetic charging field 66, and thus provides amore-uniform temperature to each of the thermistor(s) 136. In oneexample, the thermal diffuser 123 comprises a deformable layer with asticky side that can be pressed onto to the PCB 124 and over thethermistor(s) 136, such as Thermally Conductive Acrylic Interface PadPart No. 5590H, manufactured by 3M Company. In a preferred example,there are four thermistors 136, each equally placed on the PCB 124 at90-degrees inside the charging coil 126. In the example of FIG. 5E, theunderside of the PCB 124 is generally flat, and has no components orother structures applied to it. As a result, this underside may directlycontact the inner surface of bottom portion of housing 125. Note thatthe relative lack of components in charging coil assembly 102 means thateither the top or bottom of the charging coil assembly 102 may face thepatient during charging of his IMD 10.

Before discussing operation of the one or more sense coils, otheraspects of charging system 100 shown in FIG. 6A can be appreciated. Likethe external charger 50 described earlier (FIG. 3), the electronicsmodule 104 may include (as part of circuitry 112; FIG. 5A) controlcircuitry 72 that controls charging circuitry 64 to generate a chargingcurrent, Icharge. This current is passed via connector/port 108/122through a wire 134 in cable 106 to energize the charging coil 126 toproduce the magnetic field 66. The resulting voltage across the chargingcoil 126, Vcoil, perhaps as dropped in voltage using a voltage divider,can be monitored for LSK communication from the IMD 10 with theassistance of LSK demodulator 68. And again, one or more indications oftemperature (Vtherm) can be reported from the one or more thermistors136 in the charging coil assembly 102 to allow the control circuitry 72to control production of the magnetic field 66 as mentioned previously.Such conventional aspects can be used in all examples of the chargingsystem 100, and are not discussed or illustrated in subsequent examples.

While it is preferable to place control circuitry 72 and other circuitry112 aspects in the electronics module 104, this is not strictlynecessary, and instead such components can reside in the charging coilassembly 102, for example, on its circuit board 124. Thus, electronicsmodule 104 may retain only battery 110 and user interface aspects.Control circuitry 72 can comprise a microcontroller programmed withfirmware, such as any of the STM32F4 ARM series of microcontrollersprovided by STMicroeletronics, Inc., as described athttp://www.st.com/content/st_com/en/products/microcontrollers/stm32-32-bit-arm-cortex-mcus/stm32f4-series.html?querycriteria=productId=SS1577.Control circuitry 72 may also comprise an FPGA, DSP, or other similardigital logic devices, or can comprise analog circuitry at least in partas explained further below. Control circuitry 72 can further comprise amemory programmed with firmware and accessible to a microcontroller orother digital logic device should that logic device not contain suitableon-chip memory.

In a first example shown in FIGS. 6A-6C, the charging system 100includes circuitry 140 to determine a position of the charging coil 126in the charging coil assembly 102 with respect to an underlying IMD 10that is being charged. Position circuitry 140 in this example comprisespart of control circuitry 72, and thus may operate digitally asprogrammed firmware, although position circuitry 140 can also compriseanalog components as explained further below.

Charger-to-IMD positioning, as explained further below, can determinefor example whether the charging coil 126 and the IMD 10 are “aligned”or “centered.” The border between alignment and misalignment refers towhether or not the positioning between the charging coil 126 (thecharging coil assembly 102 more generally) and the IMD 10, and hencetheir coupling, is significantly poor such that the charging coil 126will no longer adequately charge the IMD's battery 14. For example, thecharging coil 126 can be said to be aligned with the IMD 10 if thecoupling value k between them is greater than 0.35, and misaligned if kis less than or equal to 0.35, although this value would be applicationspecific and could differ. Charger-to-IMD alignment is discussed priorto discussing charger-to-IMD centering.

Alignment is determined using measurements taken from sense coil 128, ofwhich there is only one in the example of FIGS. 6A-6C. Sense coil 128 isreferred to here as an “alignment” sense coil, in keeping with itsfunction of determining alignment. However, as discussed further below,other sense coils may be used in the charging coil assembly 102 fordifferent purposes.

As shown in FIG. 6B, the single alignment sense coil 128 is preferably acircle, and comprises a radius ra as measured from the center 150 of thecharging coil 126. The charging coil 126 is likewise preferablycircular, and in the example shown has a radius rp from center 150, suchthat the charging coil and the alignment sense coil are concentric.Radius rp can comprise the inside, outside, or average radius of thecharging coil 126, which coil is depicted for simplicity in FIG. 6B andsubsequent figures as just a single circle. It is preferred that radiusra be smaller than rp (e.g., that ra be between 50% to 100% of rp), asthis is useful in determining a misalignment condition, as explainedsubsequently. However, this is not strictly necessary. Radius ra mayalso equal rp, or even be greater than rp. Further, charging and sensecoils can take shapes other than circular (e.g., square, rectangular, orother shapes), although circular coils are described for simplicity.Note that even non-circular coils can share the same centers and hencebe concentric.

Radii ra and rp are also preferably set in accordance with the size ofthe IMD 10 whose battery 14 is being charged. In this regard, the IMD 10can be said to have a radius ri. Radius ri can be an estimate or averagedistance from a center 160 of the IMD 10. Center 160 can comprise acenter or centroid of the charging coil 36 (FIGS. 1B and 1C) in the IMD10, and may comprise the point that when perfectly aligned with thecenter 150 of the charging coil 126 (the charging coil assembly 102)provides a maximum coupling between the charging coils 126 and 36 in thecharging coil assembly 102 and IMD 10 respectively, and hence thefastest charging of the IMD's battery 14. Radius ri can also comprise anaverage distance between center 160 and a significant boundary of theIMD 10, such as its charging coil 36 or its case 12 or a centroid ofsuch boundary. Radii ra and rp are preferably larger than radius ri ofthe IMD 10, as this will allow the charging coil 126 to vary in positionlaterally from the IMD 10 while still keeping the IMD 10 fully boundedwithin the area of the charging coil 126. For example, it is preferredthat radii ra and/or rp be at least twice the radius of ri.

As the charging coil 126 produces the magnetic field 66, some amount ofthe magnetic field 66 will couple to the alignment sense coil 128, withthe degree of coupling being affected by position of the underlying IMD10. This coupling causes a voltage Va to be formed across the alignmentsense coil 128, which voltage will be smaller when the IMD 10 isgenerally bounded by the area of alignment sense coil 128. This is shownin the graph in FIG. 6B, which shows voltage Va as a function of radiusr, where r comprises a radial offset between the charging coil 126 andthe IMD 10, i.e., the distance between their centers 150 and 160.Voltage Va, like the magnetic field 66 and Vcoil, is AC in nature, andwill have a frequency equal to that of the magnetic field 66 coupled toit.

Va can be discussed and is graphed in FIG. 6B in terms of its maximummagnitude Va+, such as its rms, zero-to-peak, or peak-to-peak value. Asradius r increases from a perfectly centered condition (r=0), magnitudeVa+ will increase slowly as the IMD 10 is still generally bounded by thealignment sense coil 128. When radius r increases such that the IMD 10breaches the alignment sense coil 128, magnitude Va+ starts to increasemore significantly. This increase is shown linearly in FIG. 6B forsimplicity, but may be otherwise. As radius r increases further, the IMD10 will eventually be fully outside of the alignment sense coil 128, atwhich point magnitude Va+ will be maximized and again constant, as theIMD 10 will have little effect on coupling to the alignment sense coil128. Note that a more accurate graph of magnitude Va+ taken fromexperimental result is shown later in FIG. 14A.

Magnitude Va+ can be tailored by appropriate design of the alignmentsense coil 128. In this regard, note that Vcoil can range between +/−50Vor so. Va by contrast preferably varies in a range that is able to behandled by the sensing electronics in the electronics module 104, whichis explained further below. For example, Va may preferably be set tovary between +/−1V. Setting magnitude Va+ can be achieved by varying theproximity of the alignment sense coil 128 to the charging coil 126, forexample, by varying ra relative to rp. Magnitude Va+ can also be set byengineering the conductive traces in the circuit board 124 from whichalignment sense coil 128 is built. For example, the thickness and/orwidth of the traces of alignment sense coil 128 can be varied, as canthe number of turns that form the alignment sense coil 128. MagnitudeVa+ will generally scale with the number of turns. Note that whilecircular sense coil 128 appears to comprise only a single tracecomprising only a single turn in the figures for simplicity, the realitymight be otherwise, and instead a multi-turn circle-shaped sense coil128 can be formed in either single-level or multi-level trace circuitboards.

A magnitude alignment threshold, Va+(th), can be chosen from therelationship between Va+ and radius r. (A different threshold Vp+(th)shown in FIG. 6B is discussed later). In the example shown, Va+(th) canbe determined as the point at which r=ra, i.e., when the center 160 ofthe IMD 10 is located at the sense coil 128. At this point, illustratedin FIG. 6C, the IMD 10 is roughly half inside and half outside of thesense coil 128 (and the charging coil 126). This is just an examplehowever and experimentation and simulation can dictate choosing Va+(th)in different manners. For example, the alignment between the chargingcoil 126 and the IMD 10 in FIG. 6C may result in the IMD's charging coil36 receiving too little of the magnetic field 66 as a practical matter.If so, a lower value for Va+(th) could be chosen which would indicatemisalignment at a radius r smaller than ra.

Once a suitable magnitude alignment threshold Va+(th) is determined, itmay be used by position circuitry 140 to determine and indicatealignment and/or misalignment to the patient. As shown in FIG. 6A, ACsignal Va as formed across alignment sense coil 128 is sent via cable106 to the electronics module 104. Va in this example is digitized viaan Analog-to-Digital converter 142, and presented to position circuitry140. In position circuitry 140, magnitude Va+ may be determined, and howthis can occur is explained later with respect to FIGS. 13A-13D. In anyevent, position circuitry 140 can digitally compare magnitude Va+ tomagnitude alignment threshold Va+(th) stored in or accessible to theposition circuitry 140. Analog circuitry can be used as well, althoughthis isn't shown. For example, an analog magnitude Va+, for example asproduced by rectifier circuitry (not shown) could be compared to ananalog magnitude alignment threshold Va+(th) at a comparator.

Regardless of how Va+ is sensed and compared to Va+(th), the positioncircuitry 140 can issue an alignment indicator 74. For example, ifVa+>Va+(th), the position circuitry 140 can issue a misalignmentindicator 74, which can comprise issuing a sound from a speaker (a“beep”), issuing a notification on a display if the electronics module104 has one, or illuminating one of the LEDs (e.g., 118 b). Va+ may becompared to Va+(th) periodically as discussed in detail later, andmeasures of Va+ may be averaged to smooth out noise in the data. If thepatient is able to move the charging coil housing 102 to achieve betteralignment between the charging coil 126 and the IMD 10, the alignmentindicator 74 can cease. Note that alignment indicator 74 can alert thepatient to either an alignment condition, a misalignment condition, orboth.

The magnitude alignment threshold Va+(th) may be chosen assuming an IMD10 of medium depth d in a patient's tissue, and thus a medium distancebetween the charging coil 126 and the IMD 10. However, in actual use, apatient's IMD 10 may be more shallow or deeper than what is assumed, inwhich case Va+ would be decreased or increased respectively, as shown indotted lines in FIG. 6B. A single unvarying magnitude alignmentthreshold Va+(th) used in position circuitry 140 may be sufficient todetermine alignment for these different depths. On the other hand, asingle Va+(th) may also be counter-indicated because it would notintersect curves where Va is increasing at all depths, or may notintersect the curves sufficiently near a desired radius (e.g., r=ra) toaccurately establish a boundary between alignment and misalignment.Therefore, Va+(th) can be adjustable to raise Va+(th) for patientshaving deep implants and to lower Va+(th) for patients having shallowimplants so that misalignment is appropriately indicated for bothconditions. For example, the magnitude alignment threshold Va+(th) couldbe adjusted using the tuning process described in U.S. Pat. No.9,227,075 discussed in the Background.

The voltage induced across the alignment sense coil 128, Va, will varynot only as a function of IMD 10 depth, but also in accordance with thepower of the magnetic field 66 that is produced by the charging coil126. This is important to recognize because the charging coil 126 andhence the power of the magnetic field 66 may be controlled and varied inthe charging system 102 for any variety of reasons, some of which arediscussed later. Such control will also affect Va, which may makecomparison of magnitude Va+ to a set magnitude alignment thresholdVa+(th) difficult. As such, it may be necessary for the positioncircuitry 140 to normalize the Va+ measurement with respect to the powerof the magnetic field 66 before it is compared to the magnitudealignment threshold, Va+(th). Such normalization of the Va+ measurementcan comprise for example dividing Va+ by any number of parameters thatwould indicate magnetic field strength. This could include the magnitudeof the voltage across the charging coil 126 (Vcoil+), the magnitude ofthe current through the charging coil 126 (Icharge+), or inputs to thecharging circuitry 64, such as a duty cycle at which the chargingcircuitry 64 drives the charging coil 126, as explained later. Insteadof normalizing the sense coil measurement Va+, the threshold, Va+(th),could also be normalized (e.g., by multiplying it by a parameterindicative of indicate magnetic field strength).

Normalization of sense coil measurements though are not strictlynecessary. For example, the charging coil 126 may be controlled toproduce a test or default magnetic field 66 of a known constant power attimes when sense coil measurements are taken to determine alignment, asdiscussed further below. Magnitude Va+ thus would not vary due tochanges in magnetic field power at those times, allowing a magnitudealignment threshold Va+(th) to be chosen and applied with moreconfidence.

FIGS. 7A-7C show another example in which a single sense coil can beused to determine alignment. In this example, the alignment sense coil128′ comprises an edge detector coil, so named because the coil 128′ isshaped to detect the presence of the IMD 10 when it generally breachesan area (A) bounded by two circular concentric pieces relative to center150: an inner piece of smaller diameter ra1, and an outer piece oflarger diameter ra2. Alignment sense coil 128′ is circular and again isconcentric with the charging coil 126. The inner and outer pieces areconnected such that a current flowing through the alignment sense coil128′ will flow in different directions in the two pieces. For example,and as shown by the arrows in FIG. 7A, a current flowing clockwise inthe smaller diameter piece (ra1) will flow counter-clockwise in thelarger diameter piece (ra2). Ignoring the IMD 10 for a moment, noticethat a magnetic field 66 passing though the pieces of the alignmentsense coil 128′ will induce currents in the two pieces that oppose oneanother because of the manner in which they are connected. In effectthen, the total current flowing in the alignment sense coil 128′, andthe resulting voltage Va that forms across it, will be proportional tothe difference in area between the outer and inner pieces, i.e., thearea A bounded between them.

Like the first single alignment sense coil 128 of FIGS. 6A-6C, the radiiof the inner (ra1) and outer (ra2) pieces are generally close to, butpreferably smaller than, the radius rp of the charging coil 126.Further, radii ra1 and ra2 are preferably close in value (e.g., ra1between 50% to 95% of ra2) to define a narrow area A. The average radiusof the two pieces, and hence the alignment sense coil 128′ generally,may be referred to as a single radius ra for simplicity. As with thealignment sense coil 128 described earlier, each of the inner and outerpieces of alignment sense coil 128′ can be tailored in terms of theirthicknesses, lengths, and numbers of turns, although this isn'tillustrated. Again, modifying such variables is useful to tuning therange of Va. Alignment sense coil 128′ is again preferably formed in thetraces of circuit board 124, although that isn't strictly necessary.

When the charging coil 126 is perfectly aligned with the IMD 10 (i.e.,when r=0, and centers 150 and 160 coincide), the IMD 10 doesn't eclipsethe area A of the alignment sense coil 128′. The IMD 10 would thus havelimited effect on the coupling of the magnetic field 66 to the alignmentsense coil 128′, and magnitude Va+ would be near a maximum value, asshown in FIG. 7A. As radius r increases, the IMD 10 will start toencroach upon area A of the alignment sense coil 128′, and Va+ starts todecrease, eventually reaching a minimum when the IMD 10 is generallyeclipsing the alignment sense coil 128′ to a maximum extent—i.e., whenr=ra. As r increases further, the IMD 10 would eventually start to moveoutside of area A, and Va+ would increase, eventually reaching a maximumvalue when the IMD 10 no longer affects coupling to the alignment sensecoil 128′. Notice that magnitude Va+ at its maximum (high values of r)would be higher (Δ) than Va+ at low values (e.g., r=0), simply becauseat low values the IMD 10 will have some small coupling to the alignmentsense coil 128′.

The minimum value of magnitude Va+ assists in choosing a magnitudealignment threshold Va+(th) that can be used by the position circuitry140 when alignment sense coil 128′ is used. While the magnitudealignment threshold Va+(th) may be set to the minimum value of Va+,Va+(th) may also be set at a value slightly higher than this minimum toensure that it is not “missed” by the position circuitry 140. Thus, andas before, magnitude Va+ of alignment sense coil 128′ can be sensed andused to indicate misalignment by comparison to alignment thresholdVa+(th).

Position circuitry 140 may be modified to account for the difference inshape of the Va+ versus radius curve of FIG. 7A when determiningmisalignment. For example, position circuitry 140 may determine whetherVa+ has fallen (e.g., to Va(th)), and subsequently starts to increase,and issue a misalignment indicator 74 at that time. Note in this regardthat position circuitry 140 may store previous Va+ measurements as afunction of time.

Use of the edge-detection alignment sense coil 128′, and the shape ofits Va+ curve versus radius, may allow for choosing of a magnitudealignment threshold Va+(th) that can accurately establish a boundarybetween alignment and misalignment regardless of IMD 10 depth. FIG. 7Ashows two Va+ curves for deep and shallow implants. A single Va+(th) canbe chosen that is above the minimum of Va+ for both of these extremecases, hence allowing Va+ to be used to determine alignment regardlessof implant depth, and potentially without the need to adjust Va(th) fordifferent IMD depth conditions. That being said, Va+(th) can as beforebe adjusted for different IMD depths per U.S. Pat. No. 9,227,075discussed earlier. Further, either Va+ or Va+(th) can be normalized toaccount for the power of the magnetic field 66, as discussed earlier.

FIGS. 8A and 8B present another alignment sense coil arrangement 128″that is similar in function to the edge detection alignment sense coil128′ of FIGS. 7A-7C, but that uses concentric circular inner and outeralignment sense coils 128_1 and 128_2 that are not connected. As inalignment sense coil 128′, inner and outer alignment sense coils 128_1and 128_2 have radii ra1 and ra2 that are preferably close in value.However, because the alignment sense coils 128_1 and 128_2 are notconnected, they will each be induced with individual voltages Va1 andVa2 that are passed to the electronics module 104 via cable 106. Asbefore, each of alignment sense coils 128_1 and 128_2 can be tailored interms of their geometry and number of turns to achieve values for Va1and Va2 that appropriate for the electronics module 104.

At the electronics module 104, voltages Va1 and Va2 can be subtracted(or added if the voltages are of opposite polarity), which generallyequals the singular voltage Va for the edge detection alignment sensecoil 128′ of FIGS. 7A-7C. Thus, the magnitude Va+ curve againexperiences a minimum as shown in FIG. 8A, with an alignment thresholdVa+(th) being established as already discussed. Processing of the sensevoltages Va1 and Va2 can occur in position circuitry as before, with A/D142_1 and 142_2 used to digitize these voltages, and with subtraction ofthem occurring in the position circuitry 140. Alternatively, both ofvoltages Va2 and Va1 can be presented to a differential amplifier 144which can perform the subtraction prior to digitization and presentationto the position circuitry 140, as shown in dotted lines. Otherwise, theposition circuitry 140 for alignment sense coil arrangement 128″(hereinafter alignment sense coil 128″ for short, even though comprisingtwo sense coils 128_1 and 128_2) can function as before to determine andindicate misalignment. A magnitude alignment threshold Va+(th) useableby position circuitry 140 can again be adjustable based on implantdepth, and/or the position circuitry 140 can apply normalization toaccount for the power of the magnetic field 66, as explained previously.

While alignment sense coil 128″ as shown comprises two alignment sensecoils 128_1 and 128_2, note that even further numbers of alignment sensecoils 128_x could be used, such as such as three or more. The inclusionof even further numbers of alignment sense coils 128_x would providefurther information, and allow position circuitry 140 to determinealignment with further precision.

To this point, alignment sense coils 128, 128′, and 128″ have beendescribed that determine misalignment between the charging coil 126(charging coil assembly 102 more generally) and the IMD 10, i.e., whenalignment is significantly poor such that the charging coil 126 and theIMD's charging coil 36 are not well coupled, and thus the charging coil126 in its present position is unable to adequately charge the IMD'sbattery 14. However, in subsequent examples, charging system 100 usesone or more sense coils to determine whether the charging coil 126 is“centered” with respect to the IMD 10. As explained below, a chargingcoil 126 is “centered” with respect to the IMD 10 when it is wellaligned with IMD 10, i.e., when the charging coil 126 and the IMD'scharging coil 36 are very well coupled, and thus the charging coil 126is able to quickly charge the IMD's battery 14. For example, thecharging coil 126 can be said to be centered with the IMD 10 if thecoupling value k between them is greater than 0.65, and not centered ifk is less than or equal to 0.65, although again this value would beapplication specific. A charging coil 126 can thus be aligned (notmisaligned) with the IMD 10 even if it is not centered, e.g., if0.35<k<0.65).

To detect when the charging coil assembly 102 is centered with the IMD10, circuit board 124 can include one or more centering sense coils 129,shown first in FIGS. 9A-9C. In the example shown, centering sense coil129 is circular, similar to the alignment sense coil 128 of FIGS. 6A-6C,but sense coil 129 could also comprise an edge detector centering sensecoil (129′) that would be similar in geometry to the edge detectoralignment sense coil 128′ illustrated earlier in FIGS. 7A-7C. Sense coil129 could also comprise one or more separate centering sense coils(129_1 and 129_2; collectively 129″) that would be similar in geometryto the alignment sense coil 128″ illustrated earlier in FIGS. 8A and 8B.Centering sense coil is for simplicity subsequently referred to byelement 129, even though alternative non-illustrated geometries 129′ or129″ could also be used.

Like the alignment sense coils, centering sense coil 129 is induced witha voltage, Vc (or Vc1 and Vc2 if two or more separate centering sensecoils are used per 129″). Like Va, the maximum magnitude of Vc, Vc+ is afunction of the coupling to the primary charging coil 126 and couplingrelated to the proximity of the IMD 10. Thus, magnitude Vc+ will dropwhen the IMD 10 is proximate to the area encompassed by the centeringsense coil 129. Centering sense coil 129 may again be formed in theconductive traces of the circuit board 124 but could also comprise wirewindings, and the geometry of the sense coil 129 can be tailored toachieve values for Vc (or Vc1 and Vc2) that can be handled by theelectronics module 104.

Like the alignment sense coil 128, centering sense coil 129 ispreferably centered around center 150, and comprises a radius rc (orradii rc1 and rc2 with an average radius of rc if 129′ or 129″ areused). In one example, radius rc can be approximately equal to radius riof the IMD 10. As shown in the graph of magnitude Vc+ versus radius r,which shows Vc+ for a single sense coil like that of FIG. 6B earlier,Vc+ will be at a minimum when the charging coil 126 (charging coilassembly 102) is perfectly aligned with the underlying IMD 10 (i.e.,when centers 150 and 160 coincide, and r=0). As radius r increases, theIMD 10 will start to breach the extent of the centering sense coil 129almost immediately, and thus Vc+ will start to increase, and willeventually come to a maximum value when the IMD 10 is no longer coupledto the centering sense coil 129. From this graph, a magnitude centeringthreshold, Vc+(th) can be chosen. Like the magnitude alignment thresholdVa+(th) discussed earlier, Vc+(th) can be chosen in different manners.In the example shown, Vc+(th) is chosen to establish that the chargingcoil 126 is centered with respect to the IMD 10 if the radius r betweenthe two is less than ½rc. Thus, the charging coil assembly 102 will bedeemed centered to the IMD 10 so long as the center 160 of the IMD 10 iswithin a small area A′ relative to the center 150 of the charging coil126, as shown in FIG. 9B.

Positioning circuitry 140 can then compare magnitude Vc+ as measured toVc+(th) and issue a centering indication 75. For example, if thecharging coil 126 is not centered—i.e., if Vc>Vc(th)—then a centeringindicator 75 may issue, which like the alignment indicator 74 maycomprise use of a speaker, LEDs, etc. Centering indicator 75 can alertthe patient to either a centered condition, a non-centered condition, orboth. Notice also that the sensing circuitry for Vc (e.g., A/D converter142) can also be the same or similar to the circuitry used to sense Va.

Similar to the magnitude alignment threshold Va+(th), the centeringthreshold Vc+(th) could also be adjusted for individual patients basedupon the particular depth of their IMDs 10, and/or the positioncircuitry 140 can apply normalization to account for the power of themagnetic field 66, as described above.

To this point, alignment (FIGS. 6A-8B) and centering (FIGS. 9A-9C) ofthe charging coil 126 to the IMD 10 have been discussed separately.However, there are additional advantages to IMD 10 charging when bothtechniques are used together, as shown in FIGS. 10A-11B. In particular,using both techniques together allows the charging system 100 todetermine and/or indicate three possible positions of the charging coil126 relative to the IMD 10: centered (e.g., r<½rc or k>0.65), misaligned(e.g., r>ra or k<0.35), and a middle position of intermediate couplingin which the charging coil 126 is not centered but is not misalignedwith the IMD (½rc<r<ra or 0.35<k<0.65).

Starting with FIGS. 10A and 10B, the charging coil assembly 102, inaddition to charging coil 126, includes both an alignment sense coil 128and a centering sense coil 129, which can be constructed in any of thevarious forms described earlier. Single coils 128 and 129 of radii raand rc (see FIGS. 6B and 9A) are illustrated for simplicity.

The position circuitry 140 is programmed an alignment and centeringalgorithm 180, which is discussed further with reference to FIGS. 12Aand 12B. The algorithm 180 receives Va and Vc as digitized, and comparesmagnitudes Va+ and Vc+ to thresholds Va+(th) and Vc+(th) as before todetermine whether the charging coil 126 is centered, misaligned, or notcentered but not misaligned. Either or both of an alignment indicator 74and/or centering indicator 75 can issue accordingly. One skilled willrealize that algorithm 180 can be stored on any non-transitory computerreadable media, including solid state memory within the controlcircuitry 72.

One advantage of using separate alignment 128 and centering 129 sensecoils—or two different concentric coils more generally—concernsnormalization of the sense coil measurements. Va and Vc will vary withthe power of the magnetic field 66 produced by the charging coil 126,and as discussed above such measurements can be normalized to removemagnetic field power as a variable, and to make comparison to thresholdsVa+(th) and Vc+(th) more reliable. However, when two or more sense coilsare used, one sense coil measurement can be normalized using the othermeasurement, because that other measurement will be generally indicativeof magnetic field strength (even if affected by IMD coupling). Forexample, magnitude Va+ can be divided by magnitude Vc+ before it iscompared to magnitude alignment threshold Va+(th), and Vc+ can bedivided by Va+ before it is compared to centering threshold Vc+(th).

In fact, an additional sense coil can be included in the charging coilassembly 102 and measured merely for normalization purposes. Forexample, in FIG. 10B, sense coil 128 and its measured voltage Va may notnecessarily be used in an alignment determination. Instead, centeringand/or alignment might be determined by sense coil 129 and its voltageVc (as explained further with respect to FIGS. 11A and 11B), with Va+merely used to normalize Vc+ (e.g., Vc+/Va+) before it is compared to arelevant threshold (Va+(th) and/or Vc+(th)).

Alignment and centering can also be detected using a single sense coil.For example, in FIGS. 11A and 11B, a single alignment/centering sensecoil 130 is used. In the example shown, alignment/centering sense coil130 is circular, although it could comprise an edge detector sense coil(130′) or separate centering sense coils (130_1 and 130_2; collectively130″), similar to those shown in FIGS. 7A-8B. Alignment/centering sensecoil is for simplicity subsequently referred to by element 130, eventhough alternative non-illustrated geometries 130′ or 130″ could also beused.

The radius rx of the alignment/centering sense coil 130 is preferablybetween the radii ra and rc of individual alignment and centering coils128 and 129 described earlier. A magnitude of voltage Vx induced acrossthe coil 130, Vx+, can be compared to separate magnitude thresholdsVa+(th) and Vc+(th) in the alignment and centering algorithm 180 of theposition circuitry 140. Such thresholds can be chosen to establishboundaries for a centered condition (within area A′) and a misalignedcondition (outside area A″) for the charging coil 126 relative to theIMD 10. Such boundaries may coincide with or be established in light ofthe radii of the individual alignment and centering coils 128 and 129presented earlier. For example, alignment magnitude threshold Va+(th)may establish a radius of ra outside of which the IMD 10 is deemedmisaligned (i.e., when Vx+>Va+(th)), while centering magnitude thresholdVc+(th) may establish a radius ½Vc inside of which the IMD is deemedcentered (when Vx+<Vc+(th)). Alignment and centering indicators 74 and75 can again be used to indicate centered, misaligned, and/or alignedbut not centered conditions.

An example of alignment and centering algorithm 180 is summarizedbriefly before being explained in detail with respect to FIGS. 12A and12B. The algorithm 180 requires a patient at the beginning of an IMDcharging session to first center the charging coil 126 (charging coilassembly 102) with the underlying IMD 10 (e.g., r<½rc). Thereafter andduring charging, the charging coil 126 may move, and may even move froma non-centered position (e.g., r>½rc) so long as the charging coil 126is still aligned with the IMD 10 (e.g., r<ra). However, should thecharging coil 126 move to such an extent that it is no longer alignedwith the IMD 10 (e.g., r>ra), then the patient will be required to onceagain re-center the charging coil assembly 102 (e.g., r<½rc) beforecharging will again commence in earnest.

Such operation of the alignment and centering algorithm 180 isbeneficial, because it ensures, initially and later after misalignment,that the charging coil 126 is centered with the IMD 10, and thus thatthe two are very well coupled. Requiring such centered positioning meansthat the charging coil 126 is not likely to soon move out of alignmentwith the IMD 10, because it would have to move an appreciable distance(from ½rc to ra) to do so.

This is an improvement over previous alignment techniques in whichcharger-to-IMD positioning was merely assessed by a simplealigned/misaligned determination. Consider for example FIG. 3, in whichalignment was determined (70) by merely comparing Vcoil of the primarycoil 52 to an alignment threshold Vt. Assume at the beginning of acharging session that the external charger 50 is relatively poorlyaligned with the IMD 10, perhaps because it is significantly offset (x)as shown in FIG. 4B. Assume further that the external charger 50 isnonetheless still technically aligned, because Vcoil<Vt, although Vcoilis also very close to Vt. The external charger 50 could easily soon goout of alignment (Vcoil>Vt) as the patient moves. This provides afrustrating use model for the patient, who believes his external charger50 is aligned, only to find out a short time later (perhaps secondslater) that alignment requires his attention. Even thereafter should thepatient move the external charger 50 back into alignment with the IMD 10(Vcoil<Vt), the charger could again go quickly out of alignment with theIMD if again it is on the verge of being misaligned.

The alignment and centering algorithm 180 addresses this problem,because charging can't commence if the charging coil assembly 102 isonly barely aligned with the IMD 10. Instead, the charging coil assembly102 must then be centered with the IMD 10, in effect requiring the verygood alignment with the IMD 10 that centering provides.

FIG. 12A shows alignment and centering algorithm 180 in flow chart form,while FIG. 12B shows the magnetic field that is produced at the chargingcoil 126 as a result of the algorithm. First, the patient turns on thecharger system 100 (190), for example by pressing the on/off button 116(FIG. 5B) on the housing 105 of the electronics module 104. A test ordefault magnetic field can then be produced from the charging coil 126(192). As alluded to earlier, this test magnetic field 66 may be of aknown constant power, and may be lower in power that a true magneticfield 66 used later in the process to operatively charge the IMD'sbattery 14. Use of a low-power magnetic field is preferred to ensurethat the IMD 10 is not over-powered before the charging system 100 isable to determine whether the charging coil 126 is centered. A constantpower test magnetic field by using a set duty cycle with the chargingcircuitry 64, as described in detail later.

The algorithm 180 may deduce as a first step whether the IMD 10 ispresent (193)—whether the charging system 100 detects the presence ofthe IMD 10 such that assessment of position and charging can begin.Detecting the presence of the IMD 10 can occur in any number of ways.For example, the magnitude of the voltage formed across the chargingcoil 126, Vcoil+, can be assessed during production of the test magneticfield, and compared to a IMD presence magnitude threshold, Vp+(th).

Alternatively, the charging system 100 may determine IMD presence usingmeasurements taken from any of the sense coils illustrated earlier, andused later during the algorithm 180 to determine charger-to-IMDalignment and/or centering. For example, the bottom of FIG. 12A shows agraph of magnitude Va+ as measured at an alignment sense coil 128 (seeFIG. 6B). In addition to assessment of the alignment magnitude thresholdVa+(th) discussed earlier, Va+ may be compared to an IMD presencethreshold Vp+(th), which may be set just below Va+'s maximum value. IfVa+>Vp+(th), then the charging system 100 (e.g., position circuitry 140)may determine that the IMD 10 is not yet within a detectable range ofthe charging coil 126 (charging coil assembly 102). Although notillustrated, the detected presence or not of the IMD 10 may be indicatedto the user through an alert issued by the charging system's userinterface (e.g., one or more of LEDs 118 a or 118 b). Notice thereforethat the charging system 100, in addition to determining alignment andmisalignment, and/or centered and non-centered (conditions where the IMD10 is present with respect to the charging coil 126), may additionallydetermine that the IMD 10 is not present with respect to the chargingcoil 126.

Still other techniques can be used to determine IMD 10 presence withrespect to the charging coil 126 at step 193. For example, the techniqueof U.S. Pat. No. 9,186,520 can be used, which technique can also be usedto automatically turn on the charging system when IMD 10 presence isdetected.

Once IMD presence is determined and with the test magnetic field 66produced, one or more voltages are sensed to determine whether thecharging coil 126 is centered with the IMD 10 (194). For example,magnitude Vc+ of centering sense coil 129, or Vx+ of alignment/centeringsense coil 130, perhaps as normalized in the various ways describedearlier, can be compared to the centering threshold, Vc+(th). Initially,it might be expected that the charging coil 126 is not well centered,especially if the patient is in the process of placing the charging coilassembly 102 proximate to the IMD 10. In that case, the patient would bealerted (centering indication 75) that the charging coil 126 is notcentered (196) so that he can attempt to move the charging coil assembly102 into a better position relative to the IMD 10. Such alert could bein the various forms previously described. In a particular example, suchalert could comprise illuminating an LED (e.g., 118 b) on theelectronics module 104 with a color indicative of centering/alignmentstatus. For example, LED 118 b might initially be lit red before thecharging coil 126 is centered.

The assessment of centering (194) can be repeated at sensible intervals,such as every 1.0 seconds or so. Once centering is achieved—for example,when Vc+ or Vx+<Vc(th) and thus the radius r between the charging coil126 and the IMD 10 is less than ½rc as discussed in previousexamples—the previously issued non-centered alert can cease (198). Forexample, LED 118 b might now be lit green to indicate that the chargingcoil 126 is centered. Further, the charging coil 126 can now produce atrue magnetic field 66 operable to charge the IMD 10's battery 14 (200).Such true magnetic field 66 is likely be higher in power than the testmagnetic field, and may also vary per normal charging coil 126 operationand control, as shown by the arrows in FIG. 12B, and as discussedfurther subsequently. The true magnetic field 66 can continue to beproduced for a reasonable time period (t1) such as 30 second or so.

Thereafter, the alignment and centering algorithm 180 will measure thealignment of the charging coil assembly 102 with respect to the IMD 10(202). This measurement may again involve use of the constant low-powertest magnetic field. Preferably, the alignment measurement takes placequickly, for example, over a period (t2) of 1.0 seconds, and thusdoesn't significantly interrupt charging of the IMD 10 via the truemagnetic field. Misalignment can be determined by assessing themagnitude Va+ of the sense coil 128 or magnitude Vx+ ofalignment/centering sense coil 130, perhaps as normalized in the variousways described earlier.

If the charging coil 126 is not misaligned (204)—for example if Va+ orVx+ is <Va+(th) and thus the radius r is less than ra as discussed inprevious examples—then charging can then continue (200), with thecharging coil 126 once again producing a true magnetic field for anotherperiod t1. Note in this example that the charging coil 126 may no longerbe centered with the IMD 10 at this point. That is, radius r may begreater than ½rc but less than ra. But in this example, that does notmatter: so long as there is not a more-significant misalignmentcondition and thus the IMD 10 is still reasonably coupled to thecharging coil 126 and adequately charged by it, charging via the truemagnetic field can continue.

If the charging coil 126 is misaligned (206)—for example if Va+ or Vx+is >Va+(th) and thus the radius r is greater than ra—then the patient isagain alerted (alignment indicator 74), such as by lighting the LED 118b red once again. The algorithm 180 then essentially returns to itsbeginning: a test magnetic field is produced from the charging coil(192), and once again the patient must move the charging coil assembly102 to center it with the IMD 10 (e.g., r<½rc) (194-198) before a truemagnetic field can begin again (200).

As just discussed, it may not matter to the alignment and chargingalgorithm 180 that the charging coil 126 eventually becomes non-centeredwith the IMD 10, so long as it also remains aligned (202-206). Normalcharging of the IMD 10 can continue. However, optional steps inalgorithm 180 shown in dotted lines can also be used to notify the userof the non-centered but aligned condition, even if it doesn't affectcharging. If the charging coil 126 is not misaligned (204), thealgorithm 180 can nonetheless check whether it is still centered (208),similar to what occurs initially in step 194. If the still-alignedcharging coil 126 is still centered (210), charging can continue. Inthis circumstance, LED 118 b can remain green (or can be turned to greenif previously lit to amber, as explained momentarily). However, if thestill-aligned charging coil 126 is not centered (212), the user may bealerted of this fact, even though charging will still continue, and evenif the patient is not now required to move the charging coil 102. For,example, LED 118 b might be lit amber in this circumstance. In effect,these optional steps in the algorithm 180 measure three differentconditions for the charging system, and issue three different alerts: acentered condition (green), a non-centered but not misaligned condition(amber), and a misaligned condition (red).

It should be noted that steps 202 and 208—assessing alignment andcentering—need not occur in the order illustrated and could be assessedconcurrently. For example, it may be sensible to assess alignment (202)only if the charging coil 126 has become not centered (208).

Although the alignment and centering algorithm 180 is described using“test” and “true” magnetic fields 66 which may be different, this is notstrictly necessary. Instead, the charging coil 126 may produce a singletrue magnetic field 66 through operation of the entire algorithm 180 ofthe type used to meaningfully provide power to the IMD 10. Normalizingof the sense coil measurement may become more important in thiscircumstance to account for possible variations in magnetic field 66power, as discussed earlier.

To this point in the disclosure, charger-to-IMD position (e.g.,alignment and/or centering) has been determined with reference to themaximum magnitude of the voltage that is induced on the sense coil.However, the inventors realize that other sense coil parameters may beused to determine charger-to-IMD positioning, in particular the phaseangle (θ) of induced voltage relative to the signal used to drive thecharging coil 126, and the resonant frequency (f(res)) of thecharger/IMD system. The inventors realize further that use of two ormore sense coil parameters—for example, two or more of magnitude, phaseangle (θ), and resonant frequency f(res)—can be used to improve thepositioning determination, in particular by allowing charger-to-IMDdepth (d) to be determined as well as charger-to-IMD radius (r). Beforediscussing such concepts in detail, further details of the circuitry inthe charger system 100 are discussed with reference to FIGS. 13A-13C.

FIG. 13A shows further details regarding the charging circuitry 64 usedto energize the charging coil 126 with AC current, Icharge. A digitaldrive signal D is formed by a square wave generator 65, which maycomprise a part of the control circuitry 72. Drive signal D comprises apulse-width modulated (PWM) signal with a periodically-repeating portionthat is high (logic ‘1’) for a time portion ‘a’ and low for a timeportion ‘b’. As such, the drive signal D has a duty cycle DC equal toa/(a+b). Further, the drive signal D has a frequency f equal to 1/(a+b).The frequency f of the drive signal is generally set to or near theresonant frequency of the capacitor 131/charging coil 126 LC circuit(e.g., around 80 kHz), although the frequency of the drive signal canalso be adjusted, as explained subsequently.

Charging circuitry 64 can comprise a well-known H-bridge configuration,including two N-channel transistors coupled to a power supply voltageVcc, and two P-channel transistors coupled to a reference potential suchas ground (GND). The transistors are driven on and off by the drivesignal D and its logical complement D*. In so doing, the power supplyvoltage Vcc and ground are made to alternate across the LC circuit tfrequency f, thus producing the magnetic charging field 66 at thisfrequency. Power supply voltage Vcc may comprise the voltage of thebattery 110 (FIG. 5A) in the electronics module 104, or may be regulatedfrom that voltage. As is well known, the duty cycle DC of the drivesignal D can be increased from 0 to 50% to increase Icharge, thussetting the power at which the charging coil 126 is energized and hencethe power of the resulting magnetic field 66.

The AC voltage Vy (e.g., any of Va, Vc, or Vx introduced earlier)induced across the sense coil 178 (e.g., any of 128, 128′, 128″, 129,129′, 129″, 130, 130′ or 130″ introduced earlier) will also have afrequency equal to f, but may be shifted in phase angle (θ) relative tothe drive signal D (and hence relative to the voltage Vcoil across thecharging coil 126 and the magnetic field 66). This is shown in FIG. 13B,in which phase angle θ is measured as the difference between the centerof the drive signal D (top portion ‘a’) and where Vy=0. However, this isarbitrary, and phase angle θ of sense coil voltage Vy can be determinedwith respect to different reference points, or with respect to Vcoil orthe magnetic field 66.

Sense coil voltage Vy is digitized at A/D 142 as mentioned earlier, andis sampled at a frequency, Fs. As shown, the digitized samples areprovided to Vy magnitude and phase angle determination module 170, whichmay operate as firmware within the position circuitry 140 and withincontrol circuitry 72 more generally. Module 170 is capable ofconcurrently determining both the magnitude of Vy (Vy+) and the phaseangle (θ) by assessing N digitized samples of Vy. FIG. 13C explains themathematics involved. Essentially, each of the samples (Vy_(N)) ismultiplied by orthogonal trigonometric functions (sin(ω×t_(N)) orcos(ω×t_(N))), added together, and normalized by the number of samples(1/N), rendering values I and R. The magnitude of Vy (Vy+) and phaseangle (θ) are then determined as a function of I and R as shown in theequations at the right. Note that magnitude Vy+ determined in thismanner comprises a zero-to-peak value of Vy, as shown in FIG. 13B.

Magnitude Vy+ and phase angle θ could also be determined in differentmanners as one skilled will understand, and need not be determinedconcurrently in the same module or circuitry. For example, magnitude Vy+can be determined by rectifying Vy to a DC voltage, for example, using afull wave rectifier. Phase angle θ can also be determined using analogcomponents. FIG. 13D for example shows that the phase angle between Vyand drive signal D can be determined using a phase comparator 182, whichas is known outputs a voltage (Vcntr) that is indicative of the phaseangle θ. If necessary, sensed voltage Vy can be limited (clipped) andlevel shifted into a digital signal that is more easily compared to thedigital drive signal D at the phase detector 182.

It should be noted that the number of samples of Vy (N) and thefrequency at which such samples are taken (Fs) can vary depending on thedesired accuracy Vy+ and θ, and how long the measurement should take.

Another sense coil parameter that may be assessed to determinecharger-to-IMD positioning is the resonant frequency, f(res), of thecharger-IMD system. The resonant frequency f(res) can be determined in aresonant frequency determination module 172 (FIG. 13A) within theposition circuitry 140, which also can be operable as firmware. Module172 may use the phase angle θ determined in module 170 to assist indetermining the resonant frequency, f(res). Specifically, module 172 mayvary the frequency f of the drive signal D until the phase angle θreceived from module 170 equals 0, thus establishing f(res) at thatfrequency f. Such varying of the frequency can be an iterative process,and may require several adjustments to the frequency of the drive signalD. How the frequency is adjusted may involve consideration of thepolarity of the phase angle θ. For example, module 172 may lower thefrequency f if the phase angle is positive and raise it if the phaseangle is negative.

Frequency adjustment to the drive signal D to determine resonantfrequency f(res) may occur by varying either or both of time portions‘a’ and ‘b’ at the square wave generator 65. In one example, both oftime portions ‘a’ and ‘b’ may be equally scaled, thus keeping the dutycycle DC of the drive signal D constant, and thus keeping the power ofthe charging coil 126 and magnetic field 66 constant. However, this isnot strictly necessary, as variations in duty cycle and power resultingfrom small frequency adjustments may be negligible or permissible. Thus,only portions ‘a’ or ‘b’ may be varied.

Measuring f(res) of the charger-IMD system may also be accomplishedusing analog circuitry. Referring again to FIG. 13D, the determinedphase angle (expressed as Vcntr) may be compared to a reference voltage(Vref) at a comparator 186. Vref may be set to equal Vcntr when thephase angle is zero degrees, and thus the comparator may output adigital signal to the square wave generator 65 indicating whether thephase angle is higher (‘1’) or lower (‘0’) than zero degrees. The squarewave generator 65 in turn may thus increase or decrease the frequency ofthe drive signal D, which is provided back to the phase comparator 182via feedback loop 190 until Vcntr=Vref and phase angle θ equal zero, atwhich point f(res) is determined as the frequency f to which the drivesignal D has been adjusted. Still other analog feedback circuits couldbe used as well, such as phase- or delay-locked loops.

FIG. 14A shows experimental results of how the parameters of magnitudeVy+ (upper left), phase angle θ (upper right), and resonant frequencyf(res) (lower left) as measured from the sense coil 178 vary as afunction of radial offset r and depth d between centers 150 and 160 ofthe charging coil 126 and the IMD 10 respectively. FIG. 14B shows thissame data, but graphed for each sense coil parameter at a constant depth(d=10 mm), which represents a typical depth at which the charging coilassembly 102 and the IMD 10 would be separated when the IMD 10 isimplanted in a patient. In this experiment, the radius rp of thecharging coil 126 was about 30 mm, and the radius of the sense coil 178(in this example, a circle; see, e.g. FIG. 11B) was about 15-20 mm.

FIG. 14B verifies that any of sense coil parameters magnitude Vy+, phaseangle θ, or resonant frequency f(res) can be measured and assessed aloneto determine charger-to-IMD positioning—such as alignment or centeringas discussed earlier. For example, and as discussed earlier withreference to FIG. 6B for example, a magnitude threshold Vy+(th) can bestored in a database 200 associated with position circuitry 140 (FIG.13A) and compared to magnitude Vy+ as measured (module 170) to determinecharging coil 126 positioning. As the data shows, a magnitude thresholdVy+(th)=0.18 V establishes a charger-to-IMD radius r of about 27 mm atthe indicated depth (d=10 mm), which is close to the radius to thecharging coil 126 (about 30 mm). This threshold thus would work as agood determiner of charger-to-IMD alignment, such that if Vy+<Vy+(th),the charging coil 126 would be deemed aligned with the IMD 10, andmisaligned if Vy+>Vy+(th). A different threshold could also be storedand applied to determine a more-exacting centered position, such asVy+(th′)=0.14 V, which would define the charging coil 126 as centeredwhen the charging coil 126 is at or below a tighter 20 mm radius withrespect to the IMD 10. Depending on the comparison to the one or moremagnitude thresholds, the IMD position circuitry 140 may indicate (74,75) the determined position to the user, such as whether the chargingcoil 126 is centered, not centered but not misaligned, or misaligned tothe user, as explained earlier. As discussed earlier (FIGS. 10A and10B), more than one sense coil 178 concentric with the charging coil 126may also be used.

The measured resonant frequency f(res) alone can be also used todetermine charger-to-IMD positioning. As shown in FIG. 14B, a resonantfrequency threshold f(res)(th)=81.2 kHz establishes a charger-to-IMDradius r of about 27.5 mm at the indicated depth (d=10 mm), which againwould work as a good determiner of charger-to-IMD alignment: iff(res)>f(res)(th), the charger coil 126 would be deemed aligned with theIMD 10, and misaligned if f(res)<f(res)(th). A different threshold couldagain be applied to determine a more-exacting centered position, such asf(res)(th′)=81.4 kHz, which would define a tighter 20 mm centeringradius, although this additional threshold is not shown in FIG. 14B.Again, the resonant frequency f(res) as measured from more than onesense coil 178 may also be used.

The measured phase angle θ alone can also be used to determinecharger-to-IMD positioning. Notice from the data in FIG. 14B that thephase angle θ measured from the sense coil 178 lags the drive signal Dsignificantly (10°) when r=0, and falls off at higher radii, eventuallyapproaching zero degrees nearer to the radius of the charging coil 126(30 mm). A phase angle threshold θ(th) of about 0.8 degrees again worksas a suitable alignment threshold for the geometry of the charging coil126 in question and at the indicated depth. Again, and although notshown, an additional phase angle threshold could be used to determinecentering, and phase angles can be measured from more than one sensecoil 178.

Referring again to FIG. 13A, the various thresholds just described(Vy+(th), θ(th), and f(res)(th)) can be stored in database 200 andcompared to measured values, as may the three-dimensional datarepresented in FIG. 14A (Vy(r,d)), θ(r,d), f(res)(r,d)). Additionally,the depth d between the charging coil 126 and the IMD 10, which againmay vary from patient to patient, may also be stored. A priori knowledgeof depth d may allow the sense coil parameter thresholds to bedetermined with further accuracy. This can be important, because, as thecontours in FIG. 14A show, the measured parameters can vary as afunction of IMD 10 depth d, meaning that a single threshold value maynot be suitable to determine position (alignment and centering) for alldepths.

If the three-dimensional data of FIG. 14A is present, and the depth d isknown, an appropriate threshold value for each sense coil parameter canbe determined from (e.g., looked up in) database 200. For example, if itis known that the charger-to-IMD depth is 15 mm, and that misalignmentshould be indicated when the radius r exceeds 31 mm, then a suitableresonant frequency threshold f(res)(th) would be about 80.9 kHz, asshown by the dotted lines in the resonant frequency data of FIG. 14A.Charger-to-IMD depth d may also be calculated or learned and then storedin database 200, allowing appropriate thresholds to be chosen, using forexample the technique of U.S. Pat. No. 9,227,075, which technique is notdescribed here.

Variation of measured sense coil parameters with depth notwithstanding,use of phase angle θ to determine charger-to-IMD positioning isparticularly promising because of its relative insensitivity to depth d.Referring again to FIG. 14A, note that the contour for the previouslychosen threshold θ(th)=0.8 is relatively vertical, particularly at lowerdepths (d<15 mm). This means that this phase angle threshold θ(th) willwork well to determine alignment over a large depth, i.e., from 0<d<15mm. Within this depth range, the error in the established misalignmentradius, e, varies only slightly from about 25-28mm, which can betolerable.

Consideration by the IMD position circuitry 140 of more than onemeasured sense coil parameter can also allow for a determination of boththe radius r and depth d between the charging coil 126 and the IMD 10,particularly if the database 200 includes the three-dimensional data ofFIG. 14A. Assume for example that module 170 determines the magnitude ofVy, Vy+, to be 0.14 V. As the contour in FIG. 14A demonstrates, itcannot be known whether the charging coil 126 is for example at positionX1 or X2 based on Vy+ alone. However, if module 170 further determinesthat the phase angle θ is 4.0°, then position module 140 can determine(using Vy+(r,d) and θ(r,d)) that the charging coil 126 must be locatedat position X1, that is at about r=17 mm and d=10 mm relative to IMD 10(as measured relative to their centers 160 and 150).

In another example, assume that module 170 determines Vy+ to be 0.30 V.As the contour in FIG. 14A demonstrates, it cannot be known whether thecharging coil 126 is for example at position Y1 or Y2. However, ifmodule 172 further determines that the resonant frequency is 80.9 kHz,then position module 140 can determine (using Vy+(r,d) and θ(r,d)) thatthe charging coil 126 must be located at position Y1, that is, at aboutr=28 and d=18 mm relative to IMD. Consideration of a third measuredsense coil parameter (phase angle) can improve position determinationaccuracy, or verify determined position results.

Because depth d should remain relative constant for a patient, it maynot always be strictly necessary for the charging system to compute thedepth, especially considering that the depth can be learned using thedisclosed technique. For example, consideration of more than sense coilparameter can determine radius r and depth d at different points in timeduring a charging session (t2; FIG. 12B), or as the patient uses thecharging system 100 at different times. The depths at each of thesemeasurement points can be stored in database 200, and should generallynot vary. Hence, the determined depth may eventually be averaged fromthese various measurements, and then simply stored in the database 200and used without having to determine it each time sense coilmeasurements are taken.

Knowing where the charging coil 126 is relative to the IMD 10 in termsof both radius r and depth d is useful. As discussed earlier, depth d isusually fixed by the depth at which the IMD 10 is implanted in thepatient, because the charging coil assembly 102 is normally pressedagainst the patient. The patient thus may be unable to do anything toadjust the depth. By contrast, the radius between the charging coil 126and the IMD 10 is something the patient can adjust by laterally(radially) adjusting the position of the charging coil assembly 102relative to the IMD 10. Thus, the IMD position circuitry 140 preferablyadjusts the threshold(s) per IMD depth. For example, and referring tothe f(res) contours in FIG. 14A, if alignment radius is defined as r=32mm, the alignment threshold for the resonant frequency, f(res)(th),would be set to about 81.4 kHz if the depth is d=5 mm; 81.1 kHz if thedepth is d=10 mm; 80.9 kHz if the depth is d=15 mm, etc. The positionindicator (e.g., 74) would then only indicate position (in this casemisalignment) when f(res) as measured is lower than thedepth-appropriate threshold. Similar adjustment of the positionthresholds for magnitude Vy+ and phase angle θ based on depth could besimilarly determined.

Determination and indication of positioning can also occur by assessmentof more than one sense coil parameter, and comparison of more than onethreshold. For example, and again referring to the contours of FIGS.14A, misalignment might be indicated if, at depth d=10 mm, f(res)<81.1kHz and if Vy+>0.23 V, a point indicated as Z at an alignment radius ofr=32 mm. The phase angle could also be measured and considered, withmisalignment indicated or confirmed if θ<θ(th)=0.1°.

While the alignment and centering algorithm 180 described earlier withrespect to FIGS. 12A and 12B focused on use of the magnitude of thevoltage of the sense coil (Vy+), it should be noted that the algorithm180 works equally well the sense coil parameters of phase angle θ orresonant frequency f(res) are used as well. Thus, either of these othersense coil parameters could be measured and compared to appropriatethresholds (steps 194, 202, 208 of algorithm 180) to determine whetherthe charging coil 126 is centered, aligned but not centered, ormisaligned. Further, these same steps may also measure and assess morethan one of sense coil parameters Vy+, θ, and f(res), which as justexplained can improve the accuracy of the charger-to-IMD positiondetermination.

The sense coil parameters of phase angle θ and resonant frequency f(res)are less affected by the power of the magnetic field 66 provided by thecharging coil 126 during the measurement (unlike voltage magnitude Vy+,which would scale with magnetic field power). As such, normalization ofthese measurements may be unnecessary, and as such it may be unnecessaryto use a constant, lower-power test magnetic field during thosemeasurements. In other words, if θ and f(res) are used as the measuredsense coil parameters in algorithm 180, the true (possibly varying)magnetic field may be used during the measurements (e.g., during t2),and the test magnetic field can be dispensed with.

One of more of sense coil parameters Vy+, θ, and f(res) can also be usedto adjust the power of the magnetic field 66 delivered to the IMD 10.This is useful because non-ideal coupling between the charging coil 126and the IMD 10 caused by imperfect charger-to-IMD positioning may beremediable by increasing the power of the magnetic field 66 provided tothe IMD 10. In other words, if the coupling is low because ofcharger-to-IMD positioning, the power of the magnetic field is increasedto ensure that the IMD 10 receives the same amount of power no matterthe coupling.

Power control is discussed further with respect to FIG. 15A, which showsfurther details concerning power circuitry 145 useable in the chargingsystem. As with position circuitry 140 described earlier, powercircuitry 145 can operate as firmware comprising control circuitry 72,although this is not strictly necessary as analog circuitry can be usedfor certain aspects as well. Control circuitry 72 may again contain Vymagnitude and phase angle determination module 170, resonant frequencydetermination module 172, and database 200, and in FIG. 15A thosemodules have been moved out from position circuitry 140 and instead arecommon to (provide data to) both of position circuitry 140 and powercircuitry 145.

In one example, data regarding the position of the charging coil 126with respect to the IMD 10—at least the radius r, and preferably alsodepth d—is provided to power circuitry 145. As explained earlier (FIG.14A), both r and d can be determined by assessing two or more of thesense coil parameters of Vy+, θ, and f(res). However, and also asearlier explained, depth d can be programmed into the control circuitry72 and/or learned.

Power circuitry 145 can assess radius r and depth d to determine anappropriate power for the magnetic field 66. As explained earlier,magnetic field power can set by setting the duty cycle DC of the drivesignal D, because increasing the duty cycle DC will increase Ichargeflowing through the charging coil 126. However, increasing drive signalduty cycle is merely one way of increasing magnetic field power andother ways can also be used, depending on the charging circuitry 64 thatis used to energize the charging coil 126.

A look up table 146 may be stored in a memory within or accessible tothe power circuitry 145, which is used to set a duty cycle DC for thesquare wave generator 65 and the charging circuitry 64 depending on theradius and depth of the charging coil 126 relative to the IMD 10. Forexample, if the radius and depth are both relatively small (r1, d1), thecharging coil 126 and IMD 10 would be relatively well coupled. Thus,more of the magnetic field 66 generated at the charging coil 126 wouldreach the IMD 10, and as a result the duty cycle can be relatively small(DC1) for this position. By contrast, if either or both of the radiusand depth are larger (e.g., r2, d2), coupling would be poorer. Thus,power circuitry 145 would direct the square wave generator 65 toincrease the duty cycle (e.g., to DC3) to ensure that the IMD 10 isreceiving an adequate amount of power. As noted above, it is preferablethat the IMD 10 receive a constant amount of power, regardless ofcharger-to-IMD positioning.

Determining the amount of power the IMD 10 receives can beexperimentally determined in various manners by measure variousparameters in the IMD 10. In one example, the received power can beassessed as the amount of charging current, Ibat (FIG. 15A), the IMD10's battery 14 receives. One skilled will appreciate that as couplingdrops (i.e., as r or d increases), Ibat would also drop if magneticfield power were not adjusted. This is shown in FIG. 15B, which showsthe power that IMD 10 receives (Ibat) as a function of radius and depthwhen the charging coil 126 produces a magnetic field 66 of a constantpower (i.e., when Icharge and duty cycle DC are constant). Specifically,FIG. 15B shows various regions for Ibat as derived from experimentation.As shown, lower values for Ibat (e.g., between Ibat1 and Ibat2) areassociated with poor coupling, and a position region having highervalues for radius r and/or depth d. To compensate, this position regionis provided with a high duty cycle (DC5), and thus a higher magneticfield power, in look up table 146. By contrast, higher values for Ibat(e.g., >Ibat5) are associated with high coupling, and a position regionhaving lower values for radius r and/or depth d. To compensate, thisposition region is provided with a low duty cycle (DC1), and thus alower magnetic field power, in look up table 146. Preferably the dutycycle associated with each position region will cause Ibat in the IMD 10to be relatively constant, and hence independent of charger-to-IMDposition. Alternatively, the duty cycle chosen (power) may not render aconstant Ibat for all potential positions of the charging coil 126relative to the IMD 10, but will at least result in a value for Ibatthat doesn't drop below a minimum value, thus ensuring that the IMD 10receives an adequate amount of power regardless of charger-to-IMDposition.

While it is preferred that the power circuitry 145 determine anappropriate adjustments to the power of magnetic field 66 usingknowledge of both radius and depth (as determined using sense coilparameters Vy+, θ, and/or f(res)), this is not strictly necessary.Instead, power circuitry 145 may instead receive the sense coilparameters Vy+, θ, and/or f(res) themselves and adjust the power,without determining radius and depth as an intermediate step.

In one example, adjustment to the power of the magnetic field 66 occursusing sense coil measurements taken during the issuance of test ordefault magnetic field from the charging coil 126, as was discussedearlier in conjunction with FIG. 12B. In this example, one or more ofsense coil parameters Vy+, θ, and/or f(res) are periodically measuredduring the test magnetic field (e.g., during times t1)—and with thecalculated power (e.g., duty cycle DC) used during the subsequent truemagnetic field period (t2). In this way, adjustments to magnetic fieldpower are made every 30 seconds or so using the examples for t1 and t2provided earlier, which reasonably accommodates the time scale that thecharging coil assembly 102 would be expected to move. Of course, this isnot strictly necessary. Further, although not shown in FIG. 12B,additional sense coil parameters measurements could be taken (duringtime periods t2) specifically for the purpose of magnetic field poweradjustment. Thus, there can be different time periods at which sensecoil parameters measurements are taken for the purpose of determiningcharger-to-IMD positioning, and for the purpose of magnetic field poweradjustment.

Similar to what was discussed earlier in conjunction with FIG. 12B,sense coil parameter measurements for the purpose of magnetic fieldpower adjustment need not be taken using the low-powered test magneticfield, but could simply use the true magnetic field operative to chargethe IMD 10. As was discussed earlier, this may make normalizing themeasurements to the current power of the magnetic field more important,particularly the voltage magnitude Vy+ measurements, but less so for thephase angle θ and the resonant frequency f(res) measurements.

Adjustment of the power of the magnetic field using power circuitry 145can also occur as a function of the determined charger-to-IMD position.For example, power circuitry 145 may only be enabled to adjust the powerif the position circuitry 140 determines that the charging coil 126 isaligned with the IMD 10. In this regard, and as shown in FIG. 15A,position circuitry 140 may communicate the status of alignment viasignal 147 to the power circuitry 145.

The inventors realize further that it is beneficial during the provisionof power to the IMD 10 to provide a true magnetic field 66 that isoptimized at the resonant frequency of the charger/IMD system—that is, amagnetic field optimized in frequency given the mutual inductanceprovided by the coupled charger/IMD system. Providing power at theresonant frequency means that more of the power of the magnetic field 66will reach the IMD 10 and thus used to charge the IMD 10's battery 14.Further, providing power at the resonant frequency increases the signalinduced on the sense coil (Vy), thus making it easier to deducemagnitude Vy+, phase angle θ, and resonant frequency f(res). Note thatthis use of resonant frequency is different from the purpose describedearlier: while f(res) can be measured during test periods (t2) and usedto determine charger to IMD positioning and/or how to adjust magneticfield power (e.g., by duty cycle adjustment), here we refer toadjustment of the frequency of the magnetic field 66 during theprovision of a true magnetic field.

Fortunately, providing power during a true magnetic field can use thesame circuitry described earlier to measure f(res) as useful forposition determinations and power adjustment. For example, during ameasurement period (t2; see FIG. 12B), it may be determined that theresonant frequency of the charger/IMD system is f(res). As describedearlier, this f(res) measurement can be used to determine and indicate(74, 75) IMD-to-charger position, and to adjust the power of thesubsequent true magnetic field (t1), for example, by varying the dutycycle DC of the drive signal D applied to charging circuitry 64.

Further, the frequency of the drive signal may independently be set to(or allowed to continue to remain at) f(res) for the subsequent truemagnetic field period (t1) to ensure efficiency delivery of power to theIMD 10. Notice that this adjustment to the frequency of the magneticfield 66 can be independent of adjustment to its power. As describedearlier, the drive signal D has a duty cycle (power) equal to a/a+b,while its frequency is 1/a+b. Thus, while f(res) is governed by a+b, ‘a’can still be varied independently within the period of the drive signalto set the duty cycle DC.

As with power adjustment, it may be reasonable to measure f(res) duringtest periods (t2), and to set f=f(res) during the subsequent during thesubsequent true magnetic field period (t2). In this way, adjustments tothe frequency to match the resonance of the charger/IMD system are madeevery 30 seconds or so (for example). However, the frequency of thedrive signal D—and hence the frequency of the magnetic field—could alsobe adjusted on a different time scale, or adjusted pseudo continuouslyby continuously sampling sense coil voltage (and determining f(res)during production of the true magnetic field.

While the disclosed IMD position and power adjustment techniques aredescribed in the context of a charger system 100 having a separateelectronics module 104 and charging coil assembly 102 (see FIGS. 5A and5B), this is not necessary. Instead, the described techniques can alsobe implemented in an integrated external charger in which electronics,charging coil, and one or more sense coils are housed together. Forexample, FIG. 16 shows such an integrated external charger 50′ with allcomponents housed in a single housing 62, which is generally similar tothat described earlier in FIG. 2. Charger 50′ includes a circuit board54 that carries a primary charging coil 52 winding. The traces of thecircuit board 54 can include one or more sense coils 178 formed in anyof the various manners described. Control circuitry 72′ can beprogrammed with position circuitry 140, power circuitry 145, and othersupporting circuitry and programs, all of which were explained in detailabove. Thus, it is not important to the disclosed technique that thecharging/sense coils be separate from the electronics, or that they behoused in separate housings.

To this point, the disclosed sense coils can deduce charger-to-IMDposition (e.g., centered and/or aligned), but lack the ability to deducea direction by which the charging coil assembly 102 may be non-centeredor misaligned. Nonetheless, the sense coils in charging system 100 canbe modified to provide an indication to the patient a direction ofmisalignment or a non-centered condition.

For example, FIG. 17A includes two alignment sense coils 178 a and 178 bacross which voltages Vya and Vyb can be induced. In the example shown,the alignment sense coils 178 a and 178 b comprise edge detection coilsof the type disclosed earlier with respect to FIGS. 7A-7C, but this isnot strictly necessary. Further, the two illustrated coils 178 a and 178b could operate as centering sense coils, like coils 129 of FIGS. 9A-9C,or as combined alignment/centering sense coils, like coils 130 of FIGS.11A and 11B. Further, a pair of alignment sense coils and a pair ofcentering sense coils could be used, similar to that shown in FIG. 10Aand 10B. These alternatives aren't illustrated for simplicity.

It is seen in FIG. 17A that sense coils 178 a and 178 b each coverroughly half of the circumference of the PCB 124, as shown by areas Aand B. If the charging coil assembly 102 (charging coil 126) shifts suchthat the IMD 10 (not shown) eclipses area A of sense coils 178 a, thenthe magnitude Vya+ will drop. The control circuitry 72 in theelectronics module 104 would thus understand that the charging coilassembly 102 should be moved downwards to be in better alignment (orbetter centered) with the IMD 10. Likewise, if the IMD 10 eclipses areaB of sense coil 178 b, then magnitude Vyb+ will drop, indicating thatthe charging coil assembly 102 should be moved upwards. The direction ofmisalignment or non-centered, or more preferably the direction ofmovement necessary to fix alignment or centering, can be indicated bythe electronics modules user interface, and U.S. Pat. No. 8,473,066discusses various means of indication, including direction-indicatingLEDs that can be illuminated on the housing 105. The sense coilparameters of phase angle θ and resonant frequency f(res) could also bemeasured from each of sense coils 178 a and 178 b to assist determiningdirectional charger-to-IMD positioning, and magnetic field poweradjustment.

FIG. 17B is similar to FIG. 17A, but increases the areas A and Bencompassed by alignment (or sense) coils 178 a and 178 b, with eachcoil essentially covering a semicircle. In this example, inducedvoltages Vya and Vyb can both be used to determine a misalignment ornon-centered direction. For example, if Vya+ drops relatively far invalue from its maximum value, while Vyb+ only drops a small amount, thatwould indicate that the IMD 10 is eclipsing are A to a great extent, buteclipsing area B to only a small extent. Thus, the charging system 100could indicate in this example that the charging coil assembly should bemoved upward to better align or center the assembly with the IMD. Again,phase angle θ and resonant frequency f(res) could also be measured fromeach of sense coils 178 a and 178 b in FIG. 14B, and used for magneticfield power adjustment. Note that due to the non-arcuate nature of areasA and B encompassed by the sense coils 178 a and 178 b, and unlikeearlier examples of sense coils, sense coils 178 a and 178 b are notconcentric with the charging coil 126.

FIG. 17C is similar to FIG. 17B, but connects the two sense coilstogether to form a single differential sense coil 178, sometimesreferred to as a butterfly coil. In this instance, the direction ofmisalignment or non-centering would be indicated by the relativepolarity of induced voltage Vy. If Vy is significantly negative, the IMD10 would largely be eclipsing area A, while if Vy is significantlypositive, it would largely be eclipsing area B. If Vy=0, this wouldindicate that the charging coil 126 and IMD 10 are perfectly aligned.Phase angle θ and resonant frequency f(res) can also be measured fromsense coil 178 in FIG. 14C, with phase angle being particularly usefulin determining the polarity of Vy. Sense coil 178 is also not concentricwith the charger coil 126.

FIG. 17D includes sense coils 178 a-d that are similar to those in FIG.17A, but there are more than two, with each covering roughly a quarterof the circumference of the circuit board 124, as shown by areas A-D.This provides directionality information along orthogonal axes (along Xand Y directions), thus allowing the charging system to not onlydetermine whether the charging coil 126 is misaligned or not centered inthe up/down direction, but in a left/right direction as well.

FIG. 17E shows another example in which sense coils can be used incharging system 100 to deduce a direction of misalignment or anon-centered condition. In FIG. 17E, the charging coil 126 is generallyelongated (more specifically, rectangular in shape as shown), and has along dimension X that is significantly longer than its other orthogonaldimension Y. Also included within the charging coil 128 (e.g., withinPCB 124) are sense coils 178 a and 178 b, which likewise have a long Xdimension and a significantly shorter Y dimension. Two sense coils areshown, but there could be three or more. Dimension X is preferablysignificantly larger than the IMD 10. As a result, alignment between thecharging coil 126 and the IMD 10 in the X direction is generally not ofconcern, as it would be expected that the IMD 10 would be well withinthe X dimension. However, alignment in the smaller Y direction couldstill be of concern, and so sense coils 178 a and 178 b are used todetermine misalignment in the Y direction, which can be accomplished bymonitoring Vya and Vyb as explained above. Again, any one or more ofmagnitude, phase angle, and/or resonant frequency could be gleaned fromVya and/or Vyb to assist in determining alignment in the Y direction.The sense coils 178 a and 178 b, although separate as in FIG. 17B, canalso be connected together to form a single sense coil as in FIG. 17C ifdesired. Note that a charging coil 126 and sense coils 178 of the shapeshown in FIG. 17E can be used in a generally linear-shaped chargingsystem, such as that used with or in a charging belt, as mentionedearlier. Using such a charging belt, a patient would generally not haveto worry around alignment of the belt around his waist (X; in an SCSapplication), but would instead only need to worry about adjusting thebelt higher or lower on his waist (Y).

To this point, what has been assessed from the various disclosed sensecoils has been a Voltage (e.g., Va, Vc, Vx, Vy). However, sense coilcurrent could be assessed as well, with current magnitude, phase, orresonance being used in the disclosed alignment and power adjustmenttechniques.

Still other variations are possible to charging system 100. FIG. 18 forexample shows that the one or more sense coils 178 in the charging coilassembly 102 need not merely be passive devices to sense magneticfields, may also be actively energized to produce their own magneticfields to assist with alignment and/or power adjustment. As shown, sensecoil drive circuitry 65 has been added to the electronics module 104,which in this example produces an AC current, Isense, driven to sensecoil 178. Any of the sense coils disclosed earlier could be used in thisexample. The frequency of Isense may equal the resonant frequency atwhich the charging coil 126 is driven, although this isn't strictlynecessary. The frequency of Isense may be tuned in accordance with acapacitor 131 coupled to the sense coil. This capacitor 131 is shown inparallel with the sense coil, but could also be placed in series.

The one or more sense coils 178 may be driven with Isense during periodswhen the charging coil 126 is also being driven with Icharge, butpreferably would be driven during short testing periods (t2) whencharging coil 126 is not energized (t1) (see FIG. 12B). As sense coil isdriven, a magnetic field 67 is formed. A voltage Vsense will build upacross the sense coil 178, which voltage as before will be affected bycoupling to both the charging coil 126 and the position of the IMD 10relative to the sense coil. As before, Vsense will be smaller if the IMD10 is bounded by the sense coil 178, and larger if the IMD is notbounded by the sense coil. Thus, Vsense may again be compared to one ormore threshold to determine alignment, centering, and/or presence of theIMD 10, and may additionally be used to adjust the power of the magneticfield 66 generated by the charging coil 126, consistent with theprinciples explained earlier.

FIG. 19 show another variation of charging system 100 in whichtelemetered feedback from the IMD 10 is used to assist withcharger-to-IMD positioning and/or magnetic field power adjustment. Inthis example, a parameter indicative of the coupling between the primarycharging coil 126 in the charging coil assembly 102 and the secondarycharging coil 36 in the IMD 10 is telemetered to the charging system100. In one example, the coupling parameter comprises the current, Ibat,flowing into the battery during charging, which parameter can generallyscale with the extent to which the secondary charging coil 36 isreceiving the primary charging coil 126's magnetic field 66. Thiscoupling parameter however is merely one example, and other parametersmeasured in the IMD 10 could be used as well, such as the DC voltageproduced by rectifier 38.

The battery charging current Ibat can be measured by circuitry 41 in theIMD 10 in conventional fashion. For example, although not shown, Ibatcan be flow through a small sense resistor (e.g., R=1 ohm) and thevoltage across that resistor (V) measured by a differential amplifier,thus allowing the current to be deduced (Ibat=V/R). Ibat (or thecoupling parameter more generally) is received at the IMD 10's controlcircuitry 42, and can then be telemetered to the charging system 100.Such telemetry can occur in a number of ways. For example, the couplingparameter can be modulated as LSK data, where it affects Vcoil producedby the charging coil 126. Vcoil can then be demodulated (68) asexplained earlier, thus informing the charging system 100's controlcircuitry 72 of the value of the telemetered coupling parameter.

Alternatively, the coupling parameter can be telemetered via anothercommunication link established between an antenna 45 in the IMD 10 andan antenna 127 in the charging coil assembly 102. Antenna 45 maycomprise an antenna otherwise used in the IMD 10 to communicate with anexternal device such as a hand-held external controller or a clinician'sprogrammer, which external devices are explained in further detail inU.S. Patent Application Publication 2015/0360038. Antennas 45 and 127may comprise coils which communicate via near-field magnetic inductionusing an appropriate modulation scheme, such as Frequency Shift Keying(FSK). Antennas 45 and 127 may also comprise short-range RF antennasthat communicate via far-field electromagnetic waves in accordance witha communication standard such as Bluetooth, WiFi, MICS, Zibgee or otherstandards. When a discrete antenna 127 is used in the charging coilassembly 102 to receive the coupling parameter, the received data(represented as voltage Vtelem) can be reported through the cable 106 tothe control circuitry 72 in the electronics module 104, which controlcircuitry can then demodulate the data. Demodulation circuitry for theantenna 127 could also be located in the charging coil assembly 102.

Receipt of the coupling parameter at the control circuitry 72 (Vtelem)in conjunction with data reported from the one or more sense coils(Vsense) can improve the disclosed charger-to-IMD positiondetermination, and/or magnetic field power adjustment. For example, Vymight indicate that the charging coil 126 is misaligned with the IMD 10(because Vy>Vy(th)). However, if the coupling parameter indicates thatthe battery 14 in the IMD 10 is receive an adequate amount of current(Ibat), position circuitry 140 may ultimately determine that alignmentis proper. Further, if Ibat is sufficient, power circuitry 145 maydecide to not increase the power of the magnetic field 66 (e.g., theduty cycle (DC) described earlier), or may increase the power to alesser degree than may otherwise be indicated by look up table 146 (FIG.15A).

While the disclosed techniques are described in the context of a chargersystem 100 that is used to charge a battery 14 in an IMD 10, this is notstrictly necessary. Charger system 100 can also be used to providecontinuous magnetic field 66 power to an IMD that lacks a battery.Charger-to-IMD positioning and power adjustment are important in thiscontext as well, and perhaps even more so because an IMD lacking abattery may cease operating if it does not receive adequate power from apoorly positioned or non-power-optimized external charger.

Referring to “a” structure in the attached claims should be construed ascovering one or more of the structure, not just a single structure.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coverequivalents that may fall within the spirit and scope of the presentinvention as defined by the claims.

What is claimed is:
 1. An external charger for wirelessly providingenergy to an implantable medical device (IMD), comprising: a chargingcoil configured to produce a magnetic field to wirelessly provide energyto the IMD; at least one sense coil concentric with the charging coil,wherein each at least one sense coil is configured to be induced by themagnetic field with an induced signal affected by a position of thecharging coil with respect to the IMD; and control circuitry configuredto determine from a magnitude of the at least one induced signal theposition of the charging coil with respect to the IMD.
 2. The externalcharger of claim 1, further comprising an electronics module and acharging coil assembly coupled to the electronics module by a cable,wherein the charging coil and the at least one sense coil are within thecharging coil assembly, and wherein the control circuitry is within theelectronics module.
 3. The external charger of claim 1, furthercomprising a circuit board, wherein the at least one sense coil isformed in one or more traces in the circuit board.
 4. The externalcharger of claim 3, wherein the charging coil comprises a wire windingplaced on a side of the circuit board.
 5. The external charger of claim3, wherein the charging coil comprises a wire winding, and wherein thecircuit board is placed within the wire winding of the charging coil. 6.The external charger of claim 1, wherein the at least one sense coilcomprises only one sense coil.
 7. The external charger of claim 6,wherein the control circuitry is configured to determine from themagnitude whether the charging coil is aligned with or is centered withrespect to the IMD, wherein centered comprises a condition in which thecoupling between the charging coil and the IMD is higher than a firstcoupling value, and wherein aligned comprises a condition in which thecoupling between the charging coil and the IMD is higher than a secondcoupling value lower than the first coupling value.
 8. The externalcharger of claim 7, wherein the control circuitry further includes athreshold, and wherein the control circuitry is configured to determinewhether the charging coil is aligned with or is centered with respect tothe IMD by comparing the magnitude to the threshold.
 9. The externalcharger of claim 8, wherein either or both of the magnitude and thethreshold is normalized to account for a power of the magnetic field.10. The external charger of claim 6, wherein the control circuitry isconfigured to determine from the magnitude whether the charging coil isaligned with respect to the IMD, and whether the charging coil iscentered with respect to the IMD, wherein centered comprises a conditionin which the coupling between the charging coil and the IMD is higherthan a first coupling value, and wherein aligned comprises a conditionin which the coupling between the charging coil and the IMD is higherthan a second coupling value lower than the first coupling value. 11.The external charger of claim 10, wherein the control circuitry furtherincludes a first threshold and a second threshold, wherein the controlcircuitry is configured to determine whether the charging coil iscentered by comparing the magnitude to the first threshold, and whetherthe charging coil is aligned by comparing the magnitude to the secondthreshold.
 12. The external charger of claim 1, wherein the at least onesense coil comprises a first sense coil of a first radius and a secondsense coil of a second radius larger than the first radius, wherein thefirst sense coil is induced with a first induced signal with a firstmagnitude, and wherein the second sense coil is induced with a secondinduced signal with a second magnitude.
 13. The external charger ofclaim 12, wherein the control circuitry is configured to determine fromthe first magnitude whether the charging coil is centered with respectto the IMD, and from the second magnitude whether the charging coil isaligned with respect to the IMD, wherein centered comprises a conditionin which the coupling between the charging coil and the IMD is higherthan a first coupling value, and wherein aligned comprises a conditionin which the coupling between the charging coil and the IMD is higherthan a second coupling value lower than the first coupling value. 14.The external charger of claim 13, wherein the control circuitry furtherincludes a first threshold and a second threshold, and wherein thecontrol circuitry is configured to determine whether the charging coilis centered by comparing the first magnitude to the first threshold, andwhether the charging coil is aligned by comparing the second magnitudeto the second threshold.
 15. The external charger of claim 14, whereinany one or more of the first magnitude, the second magnitude, the firstthreshold, and the second threshold are normalized to account for apower of the magnetic field.
 16. The external charger of claim 1,wherein the induced signal comprises a voltage.
 17. The external chargerof claim 1, further comprising a user interface configured to indicateto a user the determined position of the charging coil with respect tothe IMD.
 18. The external charger of claim 1, wherein the charging coilcomprises a first radius, and each at least one sense coil comprises asecond radius smaller than the first radius.
 19. The external charger ofclaim 1, wherein the at least one sense coil comprises a circle.
 20. Theexternal charger of claim 1, wherein the at least one sense coilcomprises two circles of different radii.
 21. The external charger ofclaim 20, wherein the two circles are connected such that such that acurrent flowing through the two circles will flow in differentdirections in the two circles.